This invention relates to a solid state ignition system for internal combustion engines having an alternator power supply.
Capacitive discharge ignition systems for internal combustion engines provide highly reliable and satisfactory ignition and have been particularly satisfactory for two cycle engines used in recreational products such as outboard motors, snowmobiles, motor cycles and the like. Such ignition systems may employ a battery power supply with an auxiliary alternator for maintaining the battery fully charged. In smaller engines, such as employed in outboard motors or snowmobiles and the like, the alternator may be connected directly to the engine crankshaft and provide the sole power supply for the capacitive discharge ignition system. Generally, the capacitive discharge ignition system employs one or more energy storage ignition capacitors via the ignition transformers to the spark plugs of the internal combustion engine for discharge of the capacitor or capacitors at appropriate times for proper ignition of the combustible charges in the cylinders of the engine.
In some alternator driven systems, the engine flywheel is constructed with two separate sets of magnets; the first set to generate the appropriate charging voltages for the energy storage capacitor or capacitors, and the second set to generate the appropriate trigger pulses for firing of the controlled rectifiers.
In other alternator driven systems, the engine flywheel is constructed with only a single set of magnets to generate both the appropriate charging voltages and the appropriate trigger pulses.
In either type of system the trigger pulses are normally generated in a movable trigger pulse winding or coil arranged to allow angular movement both in a direction opposite to the flywheel rotation to mechanically advance the ignition timing and in a direction the same as the flywheel rotation to mechanically retard the ignition timing.
An example of a practical and highly satisfactory alternator driven system of the first type, utilizing two separate sets of magnets for the capacitor charging and the triggering, respectively, is disclosed in U.S. Pat. No. 3,805,759 which issued Apr. 23, 1974 to Arthur O. Fitzner. The triggering circuit of U.S. Pat. No. 3,805,759 includes a bias capacitor operationally in series with, and charged by, the trigger pulse winding. The output of the trigger pulse winding or generator changes with speed, and the bias capacitor is charged therefore in accordance with the speed of the engine. The capacitor is connected to effectively create a variable triggering threshold matching the varying output characteristic of the trigger pulse generator. This provides an essentially constant timing characteristic over the normal operating speed range for marine outboard motors and has been found to contribute to improved operation. The timing is, of course, then separately controlled by mechanically positioning of the trigger pulse winding means.
An example of a practical and highly satisfactory alternator driven ignition system of the second type, utilizing only a single set of magnets for both the capacitor charging and the trigger pulse generation is set forth in U.S. Pat. No. 3,937,300 which issued Feb. 10, 1976 to Richard L. Sleder et al.
Systems having only a single set of magnets as disclosed in U.S. Pat. No. 3,937,200 are advantageously employed on the smaller outboard motors which need no provisions for maintaining the charge of an external battery.
Although alternator driven capacitor discharge ignition systems provide highly satisfactory operation of internal combustion engines for outboard motors, snowmobiles and the like, smooth and particularly satisfactory engine operation may not be created under idle speed settings. The analysis of the inventors indicates that with pulse generator triggered capacitor discharge ignition systems, the timing characteristic automatically and adversely changes to some degree at the extremely low instantaneous speeds sometimes encountered at idle. This is true even with the use of the unique stabilizing circuit in U.S. Pat. No. 3,805,759. Generally, the timing characteristic, particularly at momentary speeds below the idle speed, includes an automatic retard of the spark or timing which is detrimental.
At idle, the flywheel speed varies as it rotates through one full revolution, slowing down as the engine compresses the combustible gases prior to maximum compression, perhaps speeding up very slightly as the peak of compression is passed, and them accelerating rapidly once the combustible gases are ignited.
The triggering threshold is set by the sum of the relatively fixed threshold of the controlled rectifier devices plus the variable threshold voltage contributed by the bias capacitor.
When the flywheel speed slows momentarily, as it tends to do just prior to ignition, the trigger pulse in the trigger coil, which is proportional to flywheel speed, will be weakened, and the timing will be retarded.
When the flywheel speed increases momentarily, as it tends to do immediately after ignition, the trigger pulse in the trigger coil will be strengthened and will tend to charge the bias capacitor accordingly. This will raise the variable threshold voltage in the bias capacitor and contribute to a more retarded ignition timing on the very next firing.
Overall, the result is to rapidly reduce the power output from the idling engine when the speed drops off momentarily, causing a less than desirable idle characteristic.